Quercus pacifica is endemic on three of the California Channel Islands: Santa Cruz, Santa Catalina, and Santa Rosa. It is not known from the mainland, but it bears a superficial similarity to some of the tree forms that are putative hybrids between Q . engelmannii and Q . cornelius-mulleri in San Diego County. The latter populations, sometimes treated as Q . × acutidens , differ in having much greater variability in leaf shape; thicker, more leathery leaves; denser abaxial leaf vestiture; much smaller hairs, having more than 10 rays; and variable levels of connation of cotyledons (always distinct in Q . pacifica ). Quercus pacifica appears to be most closely related to Q . douglasii , whether by direct descent or by introgression with another species no longer extant on the islands.

Quercus × macdonaldii Greene (as a species) [= Quercus dumosa var. macdonaldii (Greene) Jepson] is a stabilized hybrid complex between Quercus pacifica and Q . lobata Née. The plants tend to be small to moderate trees with leaves that resemble those of Q . lobata ; the leaves are much more shallowly lobed and always less than two-thirds the distance from the margin to the midrib. Quercus × macdonaldii is known from Santa Rosa, Santa Cruz, and Santa Catalina islands.